The present invention relates to a stator for a turbomolecular pump.
In a turbomolecular pump the rotor impellers are interleaved with stator discs fixed to spacer rings located against the inner wall of the pump housing. Although it is possible to precise the machine the inner wall of the pump housing, the periphery of the stator disks and the spacer rings, it is almost impossible to prevent formation of interstices between the outer surface of the stator and the inner wall of the pump housing. Because of the large pressure difference existing between the suction side (high vacuum side) and the delivery side (low vacuum side) of a turbomolecular pump, a back streaming gas flow can be established through such interstices from the delivery side to the suction side; that is, a stream opposite to the pumping direction which, although involving tiny gas amounts, results in a considerable worsening of the pump performance. Several approaches have been proposed to overcome this problem.
For example, in U.S. Pat. No. 4,832,564 there are provided radial ducts in the spacer rings in order to establish communication between the outside of the stator group and the inner space thereof where the pumping takes place. Since the gas conductance of these ducts is greater than that of the interstices existing between the stator group and the pump housing, the back streaming gas has a high probability of passing through the stator inner space, and thus to be pumped away.
In German Patent Application No. 2,214,702 the gas back streaming directed towards the suction side is blocked by annular seal gaskets fitted between the pump housing and the stator group.
Nevertheless these solutions are not without disadvantages due, in the first case, to the necessity of special machining of the spacer rings, and in the second case to possible sealing defects of the annular gaskets fitted between the pump housing and the stator group.
Further, the known stator discs for turbomolecular pumps comprise substantially flat structures that render troublesome the assembling between the spacer rings and do not allow of an easy positioning of each stator disc at the desired intermediate location between two adjacent impellers of the rotor. Finally, because of the construction as two separate parts of each stator disc, misalignments are possible along the diametrical division line.
The object of the present invention is to eliminate or at least to reduce the above drawbacks of the known stators for turbomolecular pumps, by providing stators that are capable of eliminating or minimizing, through simple and reliable means, the back streaming of gas from the delivery side to the suction side of the pump.
Another object of the invention is to provide stator groups wherein the assembly of each stator disc between the spacer rings is easy.
An additional, object of the invention is to provide stator discs adapted to be positioned at the optimum intermediate locations between adjacent rotor impellers.